1. Field of the Invention
The present invention relates to a radiation imaging apparatus for capturing a radiation image of an object, a method for controlling the same, and a radiation image detection device.
2. Description Related to the Prior Art
In the medical field, a radiation imaging system using radiation, for example, X-rays is known. An X-ray imaging system comprises an X-ray generating apparatus and an X-ray imaging apparatus. The X-ray generating apparatus comprises an X-ray tube for generating the X-rays. The X-ray imaging apparatus comprises an X-ray image detection device and peripheral devices such as an imaging control device and a console. The X-rays passed through an object is incident on the X-ray image detection device and thereby the X-ray image detection device detects an X-ray image representing image information of the object. The imaging control device controls the X-ray image detection device. Imaging conditions are provided to the X-ray generating apparatus. The imaging conditions include a tube current and a tube voltage. The tube current determines a radiation dose (hereinafter simply referred to as the dose) or amount of X-ray irradiation per unit time. The tube voltage determines radiation quality (energy spectrum) of the X-rays. The imaging conditions are set for each image capture, in consideration of a site (object) to be captured in an X-ray examination and age of a patient or subject, for example. The X-ray generating apparatus allows the X-ray tube to emit the X-rays in accordance with the imaging conditions.
X-ray image detection devices employing an X-ray image detector (FPD: Flat Panel Detector) instead of a conventional X-ray film or an imaging plate (IP) are in actual use (see Japanese Patent Laid-Open Publication No. 2002-301053). The FPD comprises a detection panel and a signal processing circuit. The detection panel has an image capture field in which pixels and signal lines are arranged. The pixels are arranged in matrix. Each pixel stores a signal charge in accordance with an amount of the incident X-rays. The signal line is connected to the pixels to read out the signal charges from the pixels. The signal processing circuit reads out the signal charges, stored in the pixels, as voltage signals and converts the voltage signals into digital image data. Thereby an X-ray image is viewed immediately after the image capture with the use of the X-ray image detection device employing the FPD.
In the detection panel, each pixel in the image capture field is composed of a photodiode, being a photoelectric conversion element, and a TFT (Thin Film Transistor). A scintillator (phosphor) is provided over the image capture field. The scintillator converts the X-rays into visible light. The TFT is a switching element that turns on and off the electric connection, between the photodiode and the signal line, to switch the operation of the pixel. When the TFT is turned off, the photodiode and the signal line are out of conduction. Thereby a storage operation, in which the signal charge is stored in the photodiode, is started. When the TFT is turned on, the photodiode conducts with the signal line. Thereby a readout operation, in which the signal charge is read out from the photodiode through the TFT and the signal line, is started.
The FPD differs from the X-ray film and the IP plate in that synchronous control is necessary. In the synchronous control, the start of the storage operation and the start of the readout operation are synchronized with timing of the X-ray irradiation. Examples of synchronous control methods include a method in which a synchronization signal is transmitted between the X-ray generating apparatus and the X-ray image detection device, and a method in which an X-ray image detection device measures X-ray intensity and monitors changes in the X-ray intensity to self-detect the timing of the start and the end of the X-ray irradiation.
As described in the Japanese Patent Laid-Open Publication No. 2002-301053, the X-ray image detection device allows the FPD to repeat the storage operation and the readout operation alternately at a predetermined frame rate. Thereby, moving images (fluoroscopic images) are captured using fluoroscopy or the like. During the fluoroscopy, the X-ray generating apparatus performs successive X-ray irradiation of substantially constant intensity (dose per unit time) or pulsed X-ray irradiation at a predetermined period as disclosed in Japanese Patent Laid-Open Publication No. 2006-122667.
The total dose during the fluoroscopy using the pulse irradiation is less than that using the successive irradiation because the X-rays are emitted intermittently in the pulse irradiation. The reduction in the total dose allows an increase in the intensity of the X-ray pulses in the pulse irradiation. Thus the pulse irradiation improves image quality while reducing an exposure dose of a subject. In a case where the fluoroscopy is performed using the pulse irradiation, the X-ray image detection device needs to perform synchronized control, namely, to detect timing of emission of each X-ray pulse to synchronize the timing with the storage operation of the FPD. Generally, the X-ray image detection device receives a synchronization signal from the X-ray generating apparatus to perform the synchronous control, which is called a communication method. However, it is impossible to perform the synchronous control using the communication method if the X-ray generating apparatus does not have a communication function.
The X-ray image detection device disclosed in the Japanese Patent Laid-Open Publication No. 2006-122667 is provided with a pulse detecting means. The pulse detecting means monitors changes in intensity of X-ray pulses to detect rises and falls of the X-ray pulses and thereby self-detects timing of the emissions of the X-ray pulses. The X-ray image detection device performs the synchronous control using a self detection method in which the operation of the FPD is synchronized with the emission timing detected by the pulse detecting means. The X-ray image detection device employing the synchronous control using the self detection method (hereinafter simply referred to as the synchronous control) is capable of performing the fluoroscopy using the pulse irradiation even if the X-ray image detection device cannot communicate with the X-ray generating apparatus.
However, the synchronous control has certain limits. The synchronous control cannot be performed depending on a period of an X-ray pulse, a duration of a wave tail of an X-ray pulse, or the like. FIGS. 12 and 13 show irradiation profiles each representing changes in X-ray intensity with time during the fluoroscopy using the pulse irradiation. As shown in FIG. 12, the X-ray generating apparatus starts applying a voltage when it receives a start command. When the voltage is applied, the X-ray intensity rises. The X-ray intensity reaches a peak value in accordance with a tube current and is maintained at a substantially constant value. Upon receiving a stop command, an X-ray generating apparatus stops applying the voltage and thereby the X-ray intensity falls. Thus a single X-ray pulse is generated. These steps are repeated at regular time intervals and thereby the X-ray pulses are emitted at a constant pulse period. The rise (start of the irradiation) and the fall (the end of the irradiation) of a single pulse are detected by comparing a voltage signal representing the X-ray intensity with a threshold voltage Vth. An X-ray generating apparatus with an X-ray tube composed of a commonly-used diode exhibits a relatively slow response speed to a stop command, so that time between the X-ray generating apparatus receiving the stop command and the X-ray intensity reaching “0”, that is, a duration Ts of a wave tail of the X-ray pulse, becomes long.
As shown in FIG. 12, in a case where a pulse period PP (denoted as PP1 in FIG. 12) is relatively long, the wave tail of the preceding X-ray pulse does not overlap a rising edge of the subsequent X-ray pulse even if the duration of the wave tail of the preceding X-ray pulse is long. A boundary between the two successive pulses is distinct. At a valley between the two pulses, the X-ray intensity is less than the threshold voltage Vth. Hence, the rises and the falls of the X-ray pulses are surely detected using the above-described pulse detecting means.
On the other hand, as shown in FIG. 13, in a case where the pulse period PP (denoted as PP2 in FIG. 13) is relatively short, the wave tail of the preceding X-ray pulse overlaps the rising edge of the subsequent X-ray pulse if the duration of the wave tail of the preceding X-ray pulse is long. A peak of the X-ray pulse and the valley between the X-ray pulses are indistinct. In FIG. 13, each portion with hatch lines represent an overlapping portion between the two X-ray pulses and increase in X-ray intensity due to the overlap. In the irradiation profile of FIG. 13, the X-ray intensity is maintained to be greater than the threshold value Vth due to the overlap of the X-ray pulses. This state is substantially similar to successive X-ray irradiation. In this case, the detection of the rises and the falls of the X-ray pulses and the synchronous control may not be feasible, depending on the type of the pulse detecting means.
To solve the above problem, an operator may use an X-ray generating apparatus with an X-ray tube, such as a triode or tetrode, having a fast response speed and a function to immediately attenuate the wave tail of the X-ray pulse. Thereby the overlaps between the successive X-ray pulses are eliminated and the rises and the falls of the X-ray pulses are detected. Thus, the synchronous control is performed.
However, the X-ray generating apparatus comprising the X-ray tube with the fast response speed is expensive, which increases replacement cost. Even if such X-ray generating apparatus is used, the self detection may not be allowed in a case where a frame rate of the fluoroscopy is extremely short. Thus, in some cases, the synchronous control is not feasible.
Another solution to the problem is to calculate an overlapping state of two successive X-ray pulses by an operator to determine whether the synchronous control is allowed. A duration of a wave tail of an X-ray pulse varies with a tube current, a tube voltage, and a capacity obtained in a case where the X-ray tube is considered as a resistance. The tube current and the tube voltage are changed per image capture. Hence, the calculation of the duration of a wave tail of the X-ray pulse is feasible in a case where the type of the X-ray tube, the tube current, and the tube voltage are available. The calculation of the overlapping state of the two successive X-ray pulses is feasible in a case where the calculated duration of the wave tail of the X-ray pulse and a pulse period are available. The pulse period is set in accordance with a frame rate of the fluoroscopy.
However, the calculation of the overlapping state of the X-ray pulses by an operator is extremely complicated and not practical because the calculation must be done in accordance with the tube current and the tube voltage which are changed per image capture even if the same X-ray tube is used.
Even if the X-ray tube with the fast response speed is used or the operator calculated the overlapping state of the X-ray pulses, an additional measure is necessary in a case where the synchronous control is not feasible.
The Japanese Patent Laid-Open Publication Nos. 2002-301053 and 2006-122667 do not point out explicitly or suggest the above-described problems and their solutions.